This invention relates to organic photoreceptors suitable for use in electrophotography.
In electrophotography, a photoreceptor in the form of a plate, belt, or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive layer, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and discharged areas. A liquid or solid toner is then deposited in either the charged or discharged areas to create a toned image on the surface of the photoconductive layer. The resulting visible toner image can be transferred to a suitable receiving surface such as paper. The imaging process can be repeated many times.
Both single layer and multilayer photoconductive elements have been used. In the single layer embodiment, a charge transport material and a charge generating material are combined with a polymeric binder and then deposited on the electrically conductive substrate. In the multilayer embodiment, the charge transport material and charge generating material are in the form of separate layers, each of which can optionally be combined with a polymeric binder, deposited on the electrically conductive substrate. Two arrangements are possible. In one arrangement (the xe2x80x9cdual layerxe2x80x9d arrangement), the charge generating layer is deposited on the electrically conductive substrate and the charge transport layer is deposited on top of the charge generating layer. In an alternate arrangement (the xe2x80x9cinverted dual layerxe2x80x9d arrangement), the order of the charge transport layer and charge generating layer is reversed.
In both the single and multilayer photoconductive elements, the purpose of the charge generating material is to generate charge carriers (i.e., holes and electrons) upon exposure to light. The purpose of the charge transport material is to accept these charge carriers and transport them through the charge transport layer in order to discharge a surface charge on the photoconductive element.
To produce high quality images, particularly after multiple cycles, it is desirable for the charge transport material to form a homogeneous solution with the polymeric binder and remain in solution. In addition, it is desirable to maximize the amount of charge which the charge transport material can accept (indicated by a parameter known as the acceptance voltage or xe2x80x9cVaccxe2x80x9d), and to minimize retention of that charge upon discharge (indicated by a parameter known as the residual voltage or xe2x80x9cVresxe2x80x9d).
Liquid toners generally produce superior images compared to dry toners. However, liquid toners also can facilitate stress crazing in the photoconductive element. Stress crazing, in turn, leads to printing defects such as increased background. It also degrades the photoreceptor, thereby shortening its useful lifetime. The problem is particularly acute when the photoreceptor is in the form of a flexible belt included in a compact imaging machine that employs small diameter support rollers (e.g., having diameters no greater than about 40 mm) confined within a small space. Such an arrangement places significant mechanical stress on the photoreceptor, and can lead to degradation and low quality images.
In a first aspect, the invention features an organic photoreceptor that includes:
(a) a change transport compound having the formula 
xe2x80x83where R1 and R2, independently, are hydrogen, an alkyl group (e.g., a C1-C6 alkyl group), or an aryl group (e.g., a phenyl or naphthyl group);
R3 is hydrogen or a hydrazone group having the formula 
R4 and R5, independently, are hydrogen, an alkyl group (e.g., a C1-C6 alkyl group), or an aryl group (e.g., a phenyl or naphthyl group);
R6 is an aryl group (e.g., a phenyl or naphthyl group); straight or branched alkyl group having at least 7 carbon atoms; a group having the formula xe2x80x94(CH2)nxe2x80x94Ar where n is at least 3, Ar represents an aryl group (e.g., a phenyl or naphthyl group), and one or more methylene groups is optionally substituted with a hetero atom; or a carbazole group having the formula 
where R7, R8, R9, and R10, independently, are hydrogen, an alkyl group (e.g., a C1-C6 alkyl group), or an aryl group (e.g., a phenyl or naphthyl group); n is at least 3; and one or more methylene groups is optionally substituted with a heteroatom;
(b) a charge generating compound; and
(c) an electroconductive substrate.
The organic photoreceptor may be provided in the form of a flexible belt. In one embodiment, the organic photoreceptor includes: (a) a charge transport layer comprising at least one of the charge transport compounds and a polymeric binder; (b) a charge generating layer comprising the charge generating compound and a polymeric binder; and (c) the electroconductive substrate. The charge transport layer preferably has a glass transition temperature of at least about 80xc2x0 C. In one embodiment, the charge transport layer includes two of the charge transport compounds in a ratio with one another of about 9:1 to about 1:1. The charge transport layer may be intermediate the charge generating layer and the electroconductive substrate. Alternatively, the charge generating layer may be intermediate the charge transport layer and the electroconductive substrate.
In one preferred embodiment, a charge transport compound is selected in which R3 is a hydrazone group, R4 is a methyl group, and R5 is a phenyl group. Specific examples of suitable charge transport compounds have the following formulae: 
The charge transport compounds may be used alone or in combination with each other. The invention also features the charge transport compounds themselves.
In a second aspect, the invention features an electrophotographic imaging apparatus that includes (a) a plurality of support rollers and (b) the above-described organic photoreceptor in the form of a flexible belt supported by the support rollers. Preferably, at least one of the support rollers has a diameter no greater than about 40 mm. The apparatus preferably further includes a liquid toner dispenser.
In a third aspect, the invention features an electrophotographic imaging process that includes (a) applying an electrical charge to a surface of the above-described organic photoreceptor; (b) imagewise exposing the surface of the organic photoreceptor to radiation to dissipate charge in selected areas and thereby form a pattern of charged and discharged areas on the surface; (c) contacting the surface with a liquid toner that includes a dispersion of colorant particles in an organic liquid to create a toned image; and (d) transferring the toned image to a substrate.
In a preferred embodiment, the organic photoreceptor is in the form of a flexible belt, e.g., a flexible belt supported by a plurality of support rollers, at least one of which has a diameter no greater than about 40 mm.
The invention provides organic photoreceptors featuring a combination of good mechanical properties and electrostatic properties. These photoreceptors can be used successfully with liquid toners to produce high quality images even when subjected to significant mechanical stresses encountered when the photoreceptor is in the form of a flexible belt supported by a plurality of small diameter rollers. The high quality of the images is maintained after repeated cycling.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims.
The invention features organic photoreceptors that include charge transport compounds having the formulae set forth in the Summary of the Invention, above. The charge transport compounds are aryl hydrazone-functional carbazoles. They are preferably prepared via N-alkylation of a carbazole, followed by a Vielsmayer reaction to form an aldehyde group on the carbazole molecule, and then reaction of the aldehyde with a hydrazine to form the final hydrazone product.
In some cases, it may be desirable to include two or more charge transport compounds in a single charge transport layer in order to increase solubility in the solvent used to prepare the layer. For example, a 1:1 mixture by weight of two charge transport compounds may be more soluble in a solvent such as tetrahydrofuran than each one individually at the same concentration. Increased solubility results in layers having improved uniformity and transparency, as well as improved electrostatic properties. Preferably, two charge transport compounds are utilized in a ratio with one another of about 9:1 to about 1:1 mixture by weight.
The organic photoreceptor may be in the form of a plate, drum, or belt, with the novel charge transport compounds being particularly useful in the case of flexible belts. The photoreceptor may include a conductive substrate and a photoconductive element in the form of a single layer that includes both the charge transport compound, the charge generating compound, and separate polymeric binder. Preferably, however, the photoreceptor includes a conductive substrate and a photoconductive element that is a bilayer construction featuring a charge generating layer and a separate charge transport layer. The charge generating layer may be located intermediate the conductive substrate and the charge transport layer. Alternatively, the photoconductive element may be an inverted construction in which the charge transport layer is intermediate the conductive substrate and the charge generating layer.
The photoreceptors are suitable for use in an imaging process with either dry or liquid toner development. Liquid toner development is generally preferred because it offers the advantages of providing higher resolution images and requiring lower energy for image fixing compared to dry toners. Examples of useful liquid toners are well-known. They typically include a colorant, a resin binder, a charge director, and a carrier liquid. A preferred resin to pigment ratio is 2:1 to 10:1, more preferably 4:1 to 8:1. Typically, the colorant, resin, and the charge director form the toner particles.
The photoreceptors are particularly useful in a compact imaging apparatus where the photoreceptor is wound around several small diameter rollers (e.g., having diameters no greater than about 40 mm). A number of apparatus designs may be employed, including, for example, the apparatus designs disclosed in U.S. Pat. No. 5,650,253 and U.S. Pat. No. 5,659,851.
The charge generating compound is a material which is capable of absorbing light to generate charge carriers, such as a dyestuff or a pigment One example of a suitable charge generating compound is a metal-free phthalocyanine pigment (e.g., PROGEN 1 x-form metal-free phthalocyanine pigment from Zeneca, Inc.). Also suitable are Y-form metal-free phthalocyanine pigments.
The binder is capable of dispersing or dissolving the charge transport compound (in the case of the charge transport layer) and the charge generating compound (in the case of the charge generating layer). Examples of suitable binders for both the charge generating layer and charge transport layer include styrenebutadiene copolymers, modified acrylic polymers, vinyl acetate polymers, styrene-alkyd resins, soya-alkyl resins, polyvinyl chloride, polyvinylidene chloride, acrylonitrile, polycarbonate, polyacrylic and methacrylic esters, polystyrene, polyesters, and combinations thereof Examples of suitable polycarbonate binders include aryl polycarbonates such as poly(4,4-dihydroxy-diphenyl-1,1-cyclohexane) (xe2x80x9cPolycarbonate Zxe2x80x9d) and poly(Bisphenol A carbonate co-4,4xe2x80x2(3,3,5-trimethyl cyclohexylidene diphenol)). Another example of a useful binder is polyvinyl butyral.
Other layers that may be included in the photoreceptor include, for example, barrier layers and release layers. Examples of suitable barrier layers include crosslinkable siloxanol-colloidal silica hybrids (as disclosed, e.g., in U.S. Pat. Nos. 4,439,509; 4,606,934; 4,595,602; and 4,923,775); a coating formed from a dispersion of hydroxylated silsesquioxane and colloidal silica in an alcohol medium (as disclosed, e.g., in U.S. Pat. No. 4,565,760); a polymer resulting from a mixture of polyvinyl alcohol with methyl vinyl ether/maleic anhydride copolymer; and polyvinyl butyral crosslinked with a copolymer of maleic anhydride and methylvinyl ether (commerically available under the trade designation GANTREZ AN169) containing about 30% silica. Examples of suitable release layers include fluorinated polymers, siloxane polymers, silanes, polyethylene, and polypropylene, with crosslinked silicone polymers being preferred.